Tafsir of Al-Anbiya' 21:38

(And they say: "When is this promise?"), meaning: At what time will the Hour, which has been promised, take place? They used to say this to hasten its arrival by way of mockery and denial, as indicated by the response—not out of a desire to determine its time by way of binding argument, as in Surat al-Mulk.

"Mata" (When) is in the nominative case, serving as a predicate for "hadha" (this). It has been narrated from some Kufans that it is in the accusative case, functioning as an adverb of time, with the operative agent being a verb that is implied—that is: "When will this promise come?"

(If you are truthful) in claiming that it will come. The address is directed toward the Prophet (peace be upon him) and the believers who recite the noble verses foretelling the arrival of the Hour. The apodosis (response) of the conditional clause is omitted, relying on the indication provided by what preceded it in their saying: "When is this promise?" Since their statement was an expression of impatience for the promised event and a demand for its arrival with haste, it is equivalent to a request for it to come quickly. It is as if it were said: "If you are truthful, then let it come to us quickly."